1. Field of the Invention
The present invention relates to a method and an apparatus for achieving demodulation in a radio communications system using M-sequence orthogonal modulation, more particularly relates to a method and an apparatus for achieving demodulation for a reverse-link demodulation unit in the radio communications system.
The xe2x80x9ccode division multiple access (CDMA)xe2x80x9d has come into attention as a next generation mobile communications system. In the U.S., there is already a standard system called the xe2x80x9cIS-95xe2x80x9d. Further, there is a possibility that the CDMA system used for the semifixed mobile communications system called a xe2x80x9cwireless local loop (WWL)xe2x80x9d will be used as one, means of construction of the infrastructure.
This is the CDMA with a chip rate of 1.2288 Mcps proposed by Qualcomm of the U.S. It employ a synchronous detection system using an externally inserted pilot signal for the downstream channels (forward-links). On the other hand, an asynchronous detection system using M-sequence orthogonal-modulation has been adopted for upstream channels (reverse-links).
In the IS-95 standard system, in the asynchronous detection system using the M-sequence orthogonal modulation for the upstream channels, the reception side converts an amplitude signal to electric power so as to eliminate the phase error due to fading or the like and prevent deterioration of the error rate characteristic. However, since it is an asynchronous detection system, the error rate characteristic thereof becomes worse than that of the case of a synchronous detection system.
The present invention describes method and apparatus for achieving demodulation not causing any deterioration of the error rate characteristic in a reverse-link demodulation unit operating under an asynchronous detection system using M-sequence orthogonal modulation such as in a communication system based on IS-95 (N-CDMA).
2. Description of the Related Art
As will be explained in detail later by using the drawings, in a demodulation method in a radio communications system using the M-sequence orthogonal modulation of the related art, the use of an asynchronous detection system has as a natural result the frequent occurrence of a state where both an energy of a signal of a Walsh No. 0 and an energy of a signal of a Walsh No. 35 are larger than the energy of a signal of a Walsh No. 1. For this reason, error tends to occur in the result of judgment of the Walsh number obtained from a finger demodulating unit and therefore the error rate characteristic, for example, the Eb/No vs BER characteristic, is considerably degraded. That is, the quality of demodulated data is degraded.
On the other hand, the synchronous detection system has been proposed as an alternative to the asynchronous detection system. The synchronous detection system is a system predicated on the assumption that a reception side (base station) can obtain by some means or another the phase information on a phase plane of signals of respective Walsh numbers transmitted from a transmission side (terminal) and rotates one vector shown in the figure mentioned later by exactly a predetermined phase angle corresponding to the phase information to align the vector with one axis, for example, the I-axis, and at that time rotates other vectors together by exactly that predetermined phase angle. When doing this, the situation where a large number of variety of vectors end up being concentrated at one axis (I-axis) is avoided, and the error rate characteristic is naturally improved. However, according to the synchronous detection system, there is the disadvantage of a major increase in the size of the hardware.
Accordingly, in consideration with the above problem, an object of the present invention is to provide a method and an apparatus for achieving demodulation in a radio communications system using M-sequence orthogonal modulation capable of achieving a good characteristic near the error rate characteristic obtained in the case of a synchronous detection system while suppressing an increase in the size of the hardware.
To attain the above object, according to the present invention, there is provided a demodulation apparatus having a reliability computing means (21) for computing a certainty of a value of a correlation energy as a reliability R for the correlation energy of every Walsh number, a correcting means (22) for obtaining a corrected correlation energy by introducing a corresponding reliability R with respect to the correlation energies, and a maximum energy selecting means (19) for selecting the maximum correlation energy from among obtained corrected correlation energies based on the same and demodulates a signal corresponding to a Walsh number corresponding to this maximum correlation energy as a received signal. Due to this, a demodulation apparatus in a radio communications system using M-sequence orthogonal modulation which is an asynchronous system, but can obtain a good error rate characteristic equivalent to that by the synchronous system is realized.